Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Known devices for generating signals adapted to enable calculation of a navigational indicator include conventional VHF Omni-directional Range (VOR) systems. These VOR systems originally used the phase relationship between a reference-phase and a rotating-phase signal for encoding a navigational indicator that identifies the angular direction relative to a reference direction. The carrier signal is an omni-directional signal that contains an amplitude modulated (AM) station Morse code or voice identifier. A reference 30 Hz signal is frequency modulated (FM) on a 9960 Hz sub-carrier. A second amplitude modulated (AM) 30 Hz signal is then generated by rotating a directional antenna array 30 times per second. It will be appreciated that upon receipt of the radio signal at an aircraft, a radial line from the station can be determined and followed by a pilot.
Current installations scan electronically to achieve an equivalent result. Typically called a Doppler VOR (D-VOR) system, the carrier is amplitude modulated by the reference signal and frequency modulated by the variable signal. It will be appreciated that the Doppler principle states that there is a change in frequency of a signal received when the distance between the source and receiver changes. When the distance decreases, the frequency increases. The opposite is true when the distance increases.
The D-VOR systems employ two fundamental principles: the Doppler effect for generating frequency modulated (FM) and bearing information, and a wide aperture antenna array for minimizing the effects of multipath propagation. To maintain compatibility with conventional VOR receivers, D-VOR ground station systems radiate signals with the same frequency spectrum as the conventional VOR ground stations, but the azimuth-dependent information is contained in the phase of the frequency modulated signal. For a D-VOR system, the carrier with a 30 Hz amplitude modulation is radiated from an omni-directional reference antenna and is the reference signal. The direction dependent signal is generated in space by rotating the radiated 9960 Hz sidebands from antennas located on the circumference of a circle.
The circular motion is electronically simulated by a number of antennas equally spaced around the circumference, which are sequentially driven by radio frequency (RF) signals so that a substantially continuous movement of the radiating source is achieved. The D-VOR receiver observes a Doppler shift of sideband frequencies deviating approximately at ±480 Hz thirty times a second. The D-VOR system may be a single sideband D-VOR, a doubled sideband D-VOR, or an alternating double sideband D-VOR system.
By way of example, in a D-VOR system, the reference 30 Hz signal is obtained by amplitude modulating the VHF carrier with a 30 Hz sine wave signal, and is radiated omni-directionally so that its phase is independent of the aircraft's position. The other signal has a phase that reflects the bearing around the VOR station.
As the 30 Hz signals have to be separated, a sub-carrier can be introduced. This sub-carrier, by way of example, can be in the form of two sidebands (SB1 and SB2) at 9960 Hz above and below the carrier frequency (Fc). In this example, the upper sideband (SB1) is at frequency (Fc+9960 Hz) and the lower sideband (SB2) is at frequency (Fc−9960 Hz).
It will be appreciated that the sidebands are frequency modulated, by transmitting the signals from various locations from an antenna array comprising a plurality of circumferentially located antennas, such that the transmission location rotates at 30 revolutions per second. To an observer in the far field, the frequency of the sidebands changes at a 30 Hz rate, because of the Doppler effect, and are considered to be frequency modulated at 30 Hz. The sub-carrier then space modulates the VHF carrier. The phase of the variable signal is dependant on the relative position of the receiver (aircraft) to the antenna array.
The sidebands are typically radiated from a number of circumferential sideband antennas as the location is rotated, such that SB1 and SB2 are situated diametrically opposite each other. Energy is supplied sequentially to each antenna, to simulate the rotation, and the Doppler effect provides FM modulation to the sideband.
The frequency modulation of the radiated carrier has a frequency deviation (fD) that is dependant on the diameter of the antenna array (D), rotational frequency (frot) and the wavelength of the sideband (λ). The frequency deviation can be expressed mathematically by the following equation:
      f    D    =            π      ⁢                          ⁢      D      ⁢                          ⁢              f        rot              λ  
The resulting Doppler effect produces a sinusoidal frequency modulation of the carrier frequency; the phase of which provides the bearing information.
At the receiver, the two 30 Hz signals are detected and compared to determine the phase angle between them. This phase angle is equal to the direction from the station to the airplane, typically with reference to local magnetic north.
It will be appreciated that, as a result of generating a FM signal by selectively switching between circumferential antennas, signal artefacts (or sources of distortion) are present in the combined transmission. Attempts to in part reduce these artefacts have been disclosed in the following two documents.
U.S. Pat. No. 3,972,044 discloses replacing conventional omni-directional loop antennas, typically used in circumferential antenna array, with directional antennas. These directional antennas radiate “shaped” patterns to compensate for the effect of the counterpoise.
U.S. Pat. No. 4,591,861 discloses the use of two antenna groups radiating elliptical radiation patterns, such that the radiation pattern resulting from the superposition of the two ellipses approximates a circular radiation pattern. This is regardless of receiver position and of the position of the antenna pair on the circumference. This is achieved by applying a passive network to split a portion of the signal, suitably phase shift the apportioned signals from the odd and even antennas to adjacent pairs of odd and even antennas.
Both these solutions attempt, with respect to a conventional D-VOR system, some form of fixed pre-distortion of each antenna pattern.